Our Environment

Why this chapter matters for UPSC: Chapter 15 is consistently among the top-tested science chapters in UPSC Prelims. Questions on the 10% energy flow rule, trophic levels, biomagnification, ozone layer depletion, and Montreal Protocol appear almost every year. Mains GS3 connects to biodiversity loss, plastic pollution policy, and circular economy debates.

Contemporary hook: India's single-use plastic ban (July 2022) and the global push for a Plastics Treaty (INC-5, Busan 2024) directly stem from the non-biodegradable waste problem this chapter introduces. India is now among the top producers of plastic waste, generating ~35 lakh tonnes annually (CPCB 2022).

PART 1 — Quick Reference Tables

Ecosystem Components

Energy Flow — 10% Law (Lindeman's Law)

Why only 10%? The remaining 90% is lost as heat (respiration), used in metabolic processes, and as undigested matter.

Ozone Depletion — Key Facts

Biodegradable vs Non-biodegradable

Biomagnification — Classic Example

DDT caused eggshell thinning in birds, nearly wiping out bald eagles in USA.

PART 2 — Detailed Notes

What is an Ecosystem?

Ecosystems can be natural (forest, pond, ocean, grassland, desert) or artificial (cropland, aquarium). The key feature is that energy flows and nutrients cycle within the system.

Types of ecosystems:

• Terrestrial: Forest, grassland, desert, tundra
• Aquatic: Freshwater (pond, river, lake), marine (ocean, estuary, coral reef)
• Microecosystem: A drop of pond water, a rotting log

Food Chains and Food Webs

Example food chains:

• Grass → Grasshopper → Frog → Snake → Eagle (terrestrial)
• Phytoplankton → Zooplankton → Small fish → Large fish → Dolphin (aquatic)
• Grass → Rabbit → Fox → Tiger (simple terrestrial)

Trophic Levels and Energy Flow

Trophic levels = feeding levels in a food chain.

• T1 (First trophic level) — Producers: photosynthesise, converting solar energy to chemical energy
• T2 (Second trophic level) — Primary consumers (herbivores)
• T3 (Third trophic level) — Secondary consumers
• T4 (Fourth trophic level) — Tertiary consumers

10% Law (Lindeman's Law, 1942): Only ~10% of energy is transferred from one trophic level to the next. This is why:

1. Food chains rarely exceed 4–5 links (too little energy at top)
2. Populations of top predators are always small
3. Vegetarian diets are more energy-efficient than meat-based diets

Biomagnification and Bioaccumulation

Why it happens: Non-biodegradable chemicals like DDT, methylmercury, and PCBs dissolve in fat (lipophilic) and are not excreted. As organisms eat many prey items, they accumulate the substance from all their food. Top predators (including humans who eat fish) have the highest concentrations.

Real cases:

• DDT and birds: Nearly eliminated bald eagles, peregrine falcons, pelicans in USA — banned in USA 1972; Stockholm Convention 2001 restricts DDT globally
• Minamata disease (Japan, 1950s): Methylmercury from a chemical plant bioaccumulated in fish → humans developed neurological disorders → led to the Minamata Convention 2013 on mercury
• PCBs in Arctic food web: Despite no industry nearby, polar bears have high PCB concentrations — showing global transport of persistent pollutants

The Ozone Layer

Function of stratospheric ozone: Absorbs harmful UV-B (280–315 nm) and UV-C (100–280 nm) radiation from the Sun. Without it, UV-B reaches Earth → increased skin cancer, cataracts, immune suppression, DNA damage, harm to phytoplankton and crops.

Ozone depletion mechanism:

1. CFCs released into atmosphere (from refrigerants, aerosols)
2. CFCs drift up to stratosphere — stable in troposphere
3. UV radiation breaks CFC → releases chlorine (Cl) atom
4. Cl reacts with O₃: Cl + O₃ → ClO + O₂
5. ClO reacts with O: ClO + O → Cl + O₂
6. Net result: O₃ destroyed, Cl regenerated (catalytic cycle)
7. One Cl atom can destroy 100,000 ozone molecules

Montreal Protocol (1987):

• Most successful international environmental treaty
• Binding phase-out schedule for 96 ozone-depleting substances
• All 198 UN member states have ratified it — the only UN treaty with universal ratification
• Result: Ozone layer is recovering; projected to return to pre-1980 levels by ~2040–2066

Biodegradable and Non-biodegradable Waste

Biodegradable wastes are broken down by decomposers (bacteria, fungi) into simpler inorganic substances — completing nutrient cycling. Examples: food waste, paper, cotton, wool, natural rubber, animal dung.

Non-biodegradable wastes resist decomposition. They persist in the environment for very long periods:

Plastic Pollution

Plastic is the defining non-biodegradable waste of our era:

• Macroplastics: Visible plastic litter in oceans, rivers, landfills
• Microplastics (<5mm): Fragments from degrading plastics, synthetic fibres from washing clothes, microbeads in cosmetics. Found in human blood, breast milk, placentas (recent research).
• Nanoplastics (<1μm): Can cross biological membranes

PART 3 — Frameworks & Analysis

Ecosystem Services Framework (GS3 Mains)

TEEB (The Economics of Ecosystems and Biodiversity) framework — useful for Mains answers on biodiversity valuation.

Why Ecological Balance Matters

Cascade effects when top predators are removed (trophic cascade):

• Remove wolves from Yellowstone → deer population explodes → overgrazing → riverbanks erode → fish decline → ecosystem collapse
• This is why keystone species (disproportionate ecological impact) must be protected

Ecological footprint — humans consume resources faster than ecosystems can regenerate them. Earth Overshoot Day 2024: August 1 (we used a full year's resources by August 1).

Waste Management Hierarchy (3R + 2R)

Refuse → Reduce → Reuse → Recycle → Recover

In order of environmental preference:

1. Refuse — don't buy/use unnecessary products (best option)
2. Reduce — minimise consumption
3. Reuse — use items multiple times
4. Recycle — convert waste into new materials
5. Recover — energy recovery from waste (waste-to-energy)
6. Landfill/dispose — last resort

Exam Strategy

Prelims traps:

• Ozone layer is in the stratosphere (not troposphere — ground-level ozone is a pollutant)
• Montreal Protocol = ozone, Kyoto/Paris = climate change — never mix these up
• 10% law is also called Lindeman's efficiency or ecological efficiency
• Biomagnification increases up the food chain (top predators have highest concentration)
• CFCs deplete ozone; CO₂, CH₄ cause global warming — different problems

Mains frameworks:

• On plastic pollution: Problem → Causes → Environmental impact → Policy response (India + global) → Way forward (EPR, circular economy, biodegradable alternatives)
• On ecosystem services: Provisioning + Regulating + Cultural + Supporting — with Indian examples
• On ozone: Depletion mechanism → Health and ecological effects → Montreal Protocol success → Kigali Amendment → Lessons for climate diplomacy

Previous Year Questions

Prelims:

1. Which of the following is correct regarding the ozone layer? (CSE Prelims 2015) (a) It is located in the troposphere (b) It absorbs infrared radiation (c) It protects Earth from UV radiation (d) It consists of diatomic oxygen

2. With reference to food chains in ecosystems, consider the following statements:

   1. Biomagnification of DDT occurs as energy moves up the food chain
   2. Only about 10% of energy is transferred from one trophic level to the next Which is/are correct? (a) Both 1 and 2 (b) 1 only (c) 2 only (d) Neither

Mains:

3. What are microplastics? Discuss the sources and impacts of microplastic pollution and the measures taken by India to address plastic pollution. (GS3, 15 marks)

4. What is biomagnification? Explain with an example how persistent organic pollutants (POPs) move through the food chain. What international conventions regulate POPs? (GS3, 10 marks)